Design, Installation, and Commissioning of the DØ Overpass at the Fermilab Main Ring

In order to accommodate large detectors for p-p studies at the Tevatron, the Main Ring has been modified to be non-planar. A 700 foot-long portion of the ring has been reworked to create an overpass which displaces the beam orbit upwards by 51 inches at the DØ long straight section.1 The overpass region follows the "screw" geometry proposed by T. Collins.2 A set of four vertically bending dipoles were inserted into the Main Ring lattice; they are powered on a separate bus and operate at twice the current and field level of a standard bend. To make space for these vertical bends, at each vertical bend point two of the four standard dipoles in a half-cell are removed and the other two are powered at twice the current and field level of the rest of the ring. The vertical bends also have a set of trim coils powered by a separate supply so that any difference in the horizontal and vertical bending strengths can be compensated.3 The DØ overpass was commissioned with beam in November-December 1984. The principal effect on beam dynamics - predicted and observed - is the introduction of momentum dispersion in the vertical dimension of peak value 1.9m. To preserve closed orbit quality during acceleration, the vertical bends must track the rest of the ring with a precision of better than 0.1%.